1. Field of the Invention
The present invention relates to a method of suppressing current ripple in an inverter fed motor drive system.
2. Description of the Related Art
High performance inverter fed AC motor drives require smooth torque control. The motor torque generated by an inverter fed AC drive contains various harmonic torque components.
These harmonic torque components are consequences of inverter-induced harmonics and motor-induced harmonics. Examples of inverter-induced harmonics include inverter dead-time, sensor offset, PWM chopping, etc. Examples of motor-induced harmonics include winding slots and symmetry, etc. The nth harmonic torque component mainly is generated by the (n+1)th and (nxe2x88x921)th harmonic currents interacting with the fundamental flux of the motor.
The low frequency torque ripples are considered the most crucial because they can transmit to the drive load, causing speed fluctuation, and in some cases damaging the drive load. The 6th harmonic torques, due to inverter blanking time (dead time), are of particular concern.
The consequence of inverter dead-time is torque ripple. Most of the prior art schemes address the problem by compensation of inverter voltage error. Due to the complex, non-linear behavior of the dead-time effect, however, a complete compensation of the dead-time effect may not be achieved. Moreover, the prior art schemes do not focus directly on torque ripples.
It would be beneficial to provide a direct approach to harmonic torque elimination that addresses not only the torque harmonic problem caused by inverter dead time, but also problems caused by various other imperfections, such as motor space harmonics, which lead to the generation of harmonic torques. Further, it would be beneficial to provide a method of suppressing the nth harmonic torque by eliminating the (n+1)th and (nxe2x88x921)th harmonic currents.
Furthermore, it would also be useful to be able to derive more accurate estimated motor voltages from the controller command voltages, particularly to provide better lower operating speed encoderless low cost drivers.
A method and apparatus are provided for suppressing current ripple in an inverter fed motor drive system. The method and apparatus can be used to eliminate harmonic torque currents in AC motor drive systems.
In accordance with the invention, a method is provided for regulating a harmonic current component of an inverter fed motor drive system comprising, measuring a phase current of the motor, substantially eliminating the fundamental component of the phase current, demodulating the phase current having the fundamental component substantially eliminated to produce a first current signal; and providing the first current signal to a PWM control input of an inverter drive of the inverter fed motor drive system to affect the inverter output voltage by driving a first harmonic current of the motor substantially to zero thereby to achieve harmonic current control.
Further, in accordance with the invention, apparatus is provided for regulating a harmonic current component of an inverter fed motor drive system comprising a sensor measuring a phase current of the motor, a circuit substantially eliminating the fundamental component of the phase current, a demodulator to demodulate the phase current having the fundamental component substantially eliminated to produce a first current signal and a circuit for injecting the first current signal to a PWM control input of an inverter drive of the inverter fed motor drive system to affect the inverter output voltage by driving a first harmonic current of the motor substantially to zero thereby to achieve harmonic current control.
In more detail, the harmonic torque is suppressed by a new control method that eliminates harmonic currents which contribute to the undesirable torque harmonics. The method includes measuring the motor phase currents, and removing the fundamental current component by current command. The remaining harmonic currents, which contribute to the torque ripple of concern, are discriminated by demodulation and controlled to zero by way of current regulation. For example, to eliminate the 6th harmonic torque, the (n+1)th and (Nxe2x88x921)th harmonic currents are reduced by the current regulation technique of the invention.
Outputs can be provided from the motor drive to provide more accurate estimates of motor voltage, as a result of the ripple reduction, without requiring expensive encoders.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.